Alumina coated cutting tool

Abstract: A body with a coating comprising one or more refractory layers of which at least one layer is a layer of α-Al₂O₃ textured in the (110) direction is disclosed. Said alumina layer which is essentially free of cooling cracks, comprises platelike grains with a length of 2-8 µm and a length/width-ratio of 1 - 10, preferably 3 - 7. The coated tools according to the present invention show improved tool lives compared to prior art tools when used for machining steel or cast iron material.

Description: The present invention relates to an alumina coated cutting tool for chipforming machining. Chemical Vapour Deposition (CVD) of alumina on cutting tools has been an industrial practice for more than 15 years. The wear properties of Al 2 O 3 as well as of other refractory materials have been discussed extensively in the literature. The CVD-technique has also been used to produce coatings of other metal oxides, carbides and nitrides, the metal being selected from transition metals of the IVB, VB and VIB groups of the Periodic Table. Many of these compounds have found practical applications as wear resistant or protective coatings, but few have received as much attention as TiC, TiN and Al 2 O 3 . Cemented carbide cutting tools coated with various types Al 2 O 3 -coatings, e.g., pure κ-Al 2 O 3 , mixtures of κ-and α-Al 2 O 3 and very coarse-grained α-Al 2 O 3 have been commercially available for many years. Al 2 O 3 crystallizes in several different phases: α, κ, γ, β,  etc. The two most frequently occurring phases in CVD of wear resistant Al 2 O 3 -coatings are the thermodynamically stable, hexagonal α-phase and the metastable κ-phase. Generally, the κ-phase is fine-grained with a grain size in the range 0.5-2.0 µm and often exhibits a columnar coating morphology. Furthermore, κ-Al 2 O 3 coatings are free from crystallographic defects and free from micropores or voids. The α-Al 2 O 3 grains are usually coarser with a grain size of 1-6 µm depending upon the deposition conditions. Porosity and crystallographic defects are in this case more common. Often both α- and κ-phase are present in a CVD alumina coating deposited onto a cutting tool. In commercial cutting tools, Al 2 O 3 is always applied on TiC coated carbide or ceramic substrates (see, e.g. U.S. Pat. No. 3,837,896, now Reissue U.S. Pat. No. 29,420) and therefore the interfacial chemical reactions between the TiC-surface and the alumina coating are of particular importance. In this context the TiC layer should also be understood to include layers having the formula TiC x N y O z in which the carbon in TiC is completely or partly substituted by oxygen and/or nitrogen. The practice of coating cemented carbide cutting tools with oxides to further increase their wear resistance is in itself well known as is evidenced in e.g. U.S. Pat. Reissue No. 29,420 and U.S. Pat. Nos. 4,399,168, 4,018,631, 4,490,191 and 4,463,033. These patents disclose oxide coated bodies and how different pretreatments e.g. of TiC-coated cemented carbide, enhance the adherence of the subsequently deposited oxide layer. Alumina coated bodies are further disclosed in U.S. Pat. Nos. 3,736,107, 5,071,696 and 5,137,774 wherein the Al 2 O 3 layers comprise α, κ resp α+κ combinations. US 4,619,866 describes a method for producing fast growing Al 2 O 3 layers by utilizing a hydrolysis reaction of a metal halide under the influence of a dopant selected from the groups consisting of sulphur, selenium, tellurium, phosphorous, arsenic, antimony, bismuth and mixtures thereof. A few examples of sulphur based dopants are H 2 S, COS, CS 2 , SF 6 , SF 4 , SO 2 Cl 2 and SO 2 . Under these process conditions, essentially two phases of Al 2 O 3 , the α- and the κ-phases, are produced. The resulting coating consists of a mixture of the smaller κ-grains and the larger α-grains. The process yields coatings with an even layer thickness distribution around the coated body. Swedish patent application 9101953-9 discloses a method of growing a fine grained κ-alumina coating. In Swedish Patent Application No. 9203852-0 a method is disclosed for obtaining a fine grained, (012)-textured α -Al 2 O 3 -coating. This particular Al 2 O 3 -coating applied on cemented carbide tools has been found particularly useful for cast-iron machining. Due to the difference in thermal expansion coefficient of alumina and a cemented carbide substrate, cooling cracks, forming an interconnected network, are frequently present in an Al 2 O 3 -coating, see Fig 4. During certain cutting operations with Al 2 O 3 -coated tools, the cooling cracks propagate into the cemented carbide substrate and cause premature tool failure. The cooling cracks may also initiate spotwise flaking of the entire coating structure. The object of the present invention is, thus, to provide a coated cutting tool insert with an alumina coating essentially free of cooling cracks and as a result with improved cutting performance in steel, stainless steel and in cast iron. It is further an object of the present invention to provide onto a hard substrate or preferably onto aforementioned TiC x N y O z coating at least one single phase Al 2 O 3 layer of the polymorph α with a desired microstructure and crystallographic (110)-texture using suitable nucleation and growth conditions such that said properties of the Al 2 O 3 layer are stabilized. According to the present invention there is provided a cutting tool comprising a body of a hard alloy onto which a wear resistant coating has been deposited. The coating comprises one or several refractory layers of which at least one layer is a (110)-textured Al 2 O 3 -layer of the polymorph α essentially free of cooling cracks. Figs 1 and 3 show Scanning Electron Microscope (SEM) top-view micrographs at 3,000X magnification of a typical Al 2 O 3 -coating according to the present invention, Fig 1, and according to prior art, Fig 3. Figs 2 and 4 show Scanning Electron Microscope (SEM) top-view micrographs at 200X magnification of an Al 2 O 3 -coating according to the present invention, Fig 2, and according to prior art, Fig 4. Surprisingly a method has been found to grow an α-Al 2 O 3 -coating with a specific microstructure and a crystallographic texture essentially reducing not only the number of cooling cracks but also the length and the width of each individual cooling crack. The cooling cracks are isolated and they do not form an interconnected network, (compare Fig 2 and 4). Cemented carbide tools coated with such Al 2 O 3 -layers show improved tool lives compared to prior art tools when used for machining steel or cast iron particularly if the surface has been further smoothened by wet blasting. More specifically, the present invention relates to a coated body with a thermal expansion coefficient of between 4 and 7·10 -6 K -1 provided with a <20 µm thick coating including at least one layer of alumina with a thickness of 1-10 µm and being essentially free of cooling cracks. Preferably said body is a cutting tool insert of cemented carbide, titanium based carbonitride or ceramics. The Al 2 O 3 -layer according to the present invention has a preferred crystal growth orientation in the (110) direction which is determined by X-ray Diffraction (XRD) measurements. A Texture Coefficient, TC, can be defined as: where I(hkl) = measured intensity of the (hkl) reflection I o (hkl) = standard intensity of the ASTM standard powder pattern diffraction data n = number of reflections used in the calculation, (hkl) reflections used are: (012), (104), (110), (113), (024), (116) According to the present invention, TC for the set of (110) crystal planes is larger than 1.5, preferably larger than 2.5. and most preferably larger than 3.5. The size of the Al 2 O 3 -grains varies dependent upon the nucleation and growth conditions used. The average grain size is determined from a SEM top-view micrograph at 5000X magnification. For at least ten randomly picked Al 2 O 3 -grains the length (L) and the width (W) as seen from top-view are measured. The average L and average W represent the actual grain size. For coatings according to the present invention the L/W-ratio is 1-10, preferably 3-7, with L=2-8 µm. In addition to the alumina layer(s) the coating structure comprises TiC or related carbide, nitride, carbonitride, oxycarbide and oxycarbonitride of a metal selected from the group consisting of metals in the Groups IVB, VB, and VIB of the Periodic Table, B, Al and Si and/or mixtures thereof TiC x N y O z being preferred. In a preferred embodiment the alumina layer according to the present invention is preferably the outermost layer but there may also be deposited on top of it one or more further layers. In yet another preferred embodiment said alumina layer is in contact with a TiC x N y O z -layer which preferably is the innermost layer of the coating. The textured Al 2 O 3 -coating according to the present invention is obtained according to the method of claim 6 by careful control of the oxidation potential of the CVD reactor atmosphere prior to the nucleation of Al 2 O 3 . The total concentration level of H 2 O or other oxidizing species should be below 5 ppm. However, the nucleation of Al 2 O 3 is initiated by a controlled sequencing of the reactant gases, i e the gases enter the reactor in the following order: CO 2 , CO and AlCl 3 and the temperature is within the range from about 850 to 1100°C preferably about 1000 to 1050°C. Used nucleation conditions determine the alumina polymorph, the alumina grain size and to a certain extent also the desired texture. During the deposition process a sulphur-fluorine containing gas mixture is added to the reaction mixture, preferably SF 6 , in order to obtain the desired coating microstructure. Some other possible additives are: H 2 S+F 2 , H 2 S+HF, SF 4 , CF 4 +H 2 S etc. However, the exact conditions depend to a certain extent on the design of the equipment being used. It is within the purview of the skilled artisan to determine whether the requisite texture and coating morphology have been obtained and to modify nucleation and the deposition conditions in accordance with the present specification, if desired, to affect the degree of texture and coating morphology. Example 1 A) Cemented carbide cutting inserts with a thermal expansion coefficient of 6.3·10 -6 K -1 and with the composition 6.5 % Co, 8.5 % cubic carbides and balance WC were coated with a 5.5 µm thick layer of TiCN. In subsequent process steps during the same coating cycle, a 6.5 µm thick layer of α-Al 2 O 3 was deposited. The oxidation potential of the hydrogen carrier gas, i.e. the water vapour concentration, was explicitly set forth to a low level, less than 5 ppm, prior to the Al 2 O 3 -nucleation (See also U.S. Pat. No. 5,071,696). A reaction gas mixture comprising CO 2 , CO, and AlCl 3 was sequentially added to the hydrogen carrier gas in the order specified above. The gas mixtures and other process conditions during the Al 2 O 3 deposition steps 1 and 2 comprised: Step 1 2 CO 2 : 4% 4% AlCl 3 : 4% 4% CO: 2% - SF 6 - 0.2% HCl 1% 4% H 2 : balance balance Pressure: 55 mbar 100 mbar Temperature: 1000°C 1030°C Duration: 1 hr 5.5 hr XRD-analysis showed a texture coefficient, TC(110), of 4.5 of the (110) planes in the single α phase of the Al 2 O 3 coating.SEM-studies showed that the essentially crack free α-Al 2 O 3 -coating consisted of plate-like grains with an average grain size of L = 3.5 µm and W= 0.7 µm. However, the very few occurring cracks had a crack width of approximately 0.01 µm and an average length of about 20 µm. B) The cemented carbide substrate of A) was coated with TiCN (5.5 µm) and Al 2 O 3 (6.5 µm) as set forth in A) except for that the Al 2 O 3 process was carried out according to prior art technique resulting in a mixture of coarse α- and fine κ-Al 2 O 3 grains in the coating. The coating showed a well developed crack pattern with an average crack distance of 100 µm and a crack width of approximately 0.05 µm. Coated tool inserts from A) and B) were all wet blasted with 150 mesh Al 2 O 3 powder in order to smoothen the coating surface. The cutting inserts were then tested with respect to edge line and rake face flaking in a facing operation in nodular cast iron (AISI 60-40-18, DIN GGG40). The shape of the machined work piece was such that the cutting edge is intermitted twice during each revolution. Cutting data: Speed = 150 m/min, Cutting Depth = 2.0 mm Feed = 0.1 mm/rev. The inserts were run one cut over the face of the work piece. The results are expressed in the table below as percentage of the edge line in cut that obtained flaking as well as the rake face area subjected to flaking in relation to total contact area between the rake face and the work piece chip. Flaking (%) Edge line Rake face A) single phase/textured α-Al 2 O 3 0 0 (acc. to the invention) B) α+κ Al 2 O 3 45 8 Example 2 The cutting inserts from A) and B) were also tested with respect to edge line flaking in a facing operation in an alloyed steel (AISI 1518, W-no. 1,0580). The shape of the machined work piece was such that the cutting edge was intermitted three times during each revolution. Cutting data: Speed = 130-220 m/min, Cutting Depth = 2 mm Feed = 0.2 mm/rev. The inserts were run one cut over the face of the work piece. The result below is expressed as percentage of the edgeline in cut that obtained flaking. Flaking (%) Edge line D) single phase/textured α-Al 2 O 3 0 (according to the invention) E) α+κ Al 2 O 3 35

Claims: Body with a <20 µm thick coating including at least one layer of alumina with a thickness of 1-10 µm and being essentially free of cooling cracks, said body having a thermal expansion coefficient between 4 and 7·10 -6 K -1 characterized in that said alumina layer consists of single phase α-structure textured in the (110)-direction with a texture coefficient larger than 1.5, the texture coefficient being defined as below: where I(hkl) = measured intensity of the (hkl) reflection I o (hkl) = standard intensity of the ASTM standard powder pattern diffraction data n = number of reflections used in the calculation (hkl) reflections used are: (012), (104), (110), (113), (024), (116) at which said alumina layer comprises platelike grains with a length of 2-8 µm and a length/width-ratio of 1 - 10. Body according to the preceding claim, characterized in that said alumina layer is the outermost layer. Body according to any of the preceding claims characterized in that said alumina layer is in contact with a TiC x N y O z -layer. Body according to claim 3 characterized in that said TiC x N y O z -layer is the innermost layer of the coating. Body according to any of the preceding claims characterized in that said body is a cutting tool insert of cemented carbide, titanium based carbonitride or ceramics. Method of coating a body with the textured alpha-alumina coating according to claim 1 by which the body is brought in contact with a hydrogen carrier gas containing one or more halides of aluminium and a hydrolyzing and/or oxidizing agent at high temperature characterized in that the oxidation potential of the CVD reactor atmosphere prior to the nucleation of Al 2 O 3 is kept at a low level using a total concentration level of H 2 O or other oxidizing species below 5 ppm, that the nucleation of Al 2 O 3 is started up by a controlled sequencing of the reactant gases in the following order: CO 2 , CO and AlCl 3 , that the temperature is about 850 to 1100°C during the nucleation and that during the growth of the Al 2 O 3 -coating a sulphur-fluorine containing gas mixture is added to the reaction mixture.

Inventors: LJUNGBERG BJOERN, LJUNGBERG, BJOERN, Ljungberg, Björn
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1994
Main classification: C23C16
IPC codes: B23P15, C23C16, C23C30, B23B27, C23C28